The objective of the Program on Surgical Control of the Hyperlipidemias (POSCH) is to prove, or disprove, the lipid-atherosclerosis hypothesis by maximal cholesterol reduction in an unifactorial secondary intervention trial of 1,000 subjects, in which data analysis includes objective and quantitative assessment of coronary atherosclerotic plaque lesions. Maximal cholesterol reduction is defined as the cholesterol lowering achieved by the partial ileal bypass operation. This procedure has been proven to be safe, lasting, and, of course, obligatory - i.e., having its lipid effects free from patient influence. The lipid data derived from the POSCH trial itself, with both the control and operated groups on a cholesterol-lowering diet, have documented the total cholesterol concentration to be lower in the partial ileal bypass group, accompanied by a reduction in the LDL-cholesterol level. Unifactorial trial is defined as a trial of lipid reduction with, as far as possible, freedom from confounding risk factors by exclusion. Secondary intervention is intervention in the atherosclerosis progress after that process has progressed to overt disease, as manifested by a single, well-documented myocardial infarction. The POSCH trial, in addition to the traditional assessment of clinical endopoints and events, and the evaluation of sequential graded stress electrocardiograms employs computerized reading and weighted numberical comparisions of sequential peripheral and coronary arteriograms.